1. Field of the Invention
The present invention relates generally to injection molding and more specifically to a screw design for an injection molding machine configured for use with processing metals and method of configuring an injection molding machine for use with processing metals.
2. Background of the Related Art
Processing metals into 3-dimensional net shapes via conventional reciprocating screw injection molding equipment used for plastics has been a long time goal of many research efforts. Injection molding is a low cost processing technique to produce complex parts but has been limited to the molding of plastics for a variety of reasons.
There is an extremely large installed based of injection molding equipment worldwide. It is difficult to define the exact number but it is likely that there are more than 1 million injection molding machines in commercial use today. As an example, deliveries of injection molding machines in China alone have averaged around 50,000 units/year for the last few years. Injection molding equipment has a finite lifetime but it is a minimum of 10 years and many machines operate for 20 years or greater especially if they are properly maintained and/or upgraded (e.g. electronics).
Metals are generally accepted as not processable in conventional injection molding equipment intended for plastic processing. There are two main reasons. First, metal and their alloys of commercial interest generally (there are exceptions) have melting temperatures that are significantly above the maximum temperature capability of the majority of injection molding machines (typically 400° C./˜800° F.). This temperature is sufficient for all or almost all organic polymers since they all tend to start to degrade (e.g. oxidize, carbonize, decompose) at temperatures above 400° C. (˜800° F.).
The second issue is pressure. Although molten metals above their liquidus temperature have a very low viscosity, they crystallize so rapidly that it is difficult to overcome the strength of crystal formation in injection molding equipment. On the other hand, polymers (amorphous polymers and semi-crystalline polymers to a lesser extent) are viscous materials with a broad viscosity versus temperature relationship. Therefore, flow can be controlled by a combination of temperature and pressure. Unlike most metals, viscosity in polymers never drops to an extremely low value (e.g. water-like) such that it would be difficult to control
The requirement for a material to have a finite force below which it will not move is an important characteristic for processing utility in conventional injection molding equipment. Polymers generally meet these criteria. Metals, in general, have a much sharper transition at their melting point. There are exceptions in including semi-solid metals (semi molten metals at a temperature intermediate to their liquidus and solidus temperatures) and amorphous metal alloys that have a composition that retards or delays crystal formation.
As a result, the generally excepted method for producing 3-dimensional net shape parts from metals is die casting. In die casting the process temperature is well above the liquidus temperature and the molten metal is poured by gravity or pressure assisted to fill a cavity. Die casting and pressure assisted die casting are accepted processing methods and there are a large number of die casting facilities and equipment worldwide. There are some disadvantages to die casting based primarily on the uncontrolled flow of the material while filling the cavity. The lack of rheological control on the flow (water like viscosity) cause mold filling that is inconsistent, often causes voids or defects, creates undesirable surface finish effects, and less than desirable dimensional control (shrinkage). Another approach is to work with the metal in the semi-solid state (between its liquidus and solidus temperatures) to effectively lower the process temperature. Cooling of the semi-solid also produces lower shrinkage because of a portion of the “melt” is already solidified. This approach is used in the molding of certain magnesium alloys using a modified injection molding process referred to as thixomolding. One of the drawbacks of either of these processes is the availability of commercial equipment. Die casting usually involves a foundry-like environment to reach the process temperatures required. Thixomolding requires somewhat lower temperatures but uses force and therefore, very robust and specialized equipment, to overcome the rapid solidification or crystal growth. Thixomolded parts also in general have significant secondary requirements, surface finish repair, flash removal. There are also significant requirements on the handling and reprocessing of scrap, runners, etc.
A third route to near net shape metallic parts is often referred to as metal injection molding (MIM) or powder injection molding (PIM). In this case a perform or green part is injection molded at conventional temperatures using powdered metals and an organic or polymeric binder. The binder is removed and the part is sintered at high temperature in a reducing environment to generate the part. A large volume reduction (shrinkage) is associated with the sintering step. A fourth route is machining of the part from larger shapes or ingots to generate the desired dimensions. Additional methods (e.g. forging) can create some 3-dimensional shape but are not suitable for complex structures.
The four processes described are all used commercially and successfully. Yet, they all have significant cost or other drawbacks that limit wider utility and commercial significance. It would be certainly desirable if metal alloys were processable into 3-dimensional net shape parts using conventional injection molding equipment.
Therefore, there is a need within the industry for a method of processing metals in injection molding equipment.